half wave rectification and Ge diodes

Frequent question abouts LED and other types of diodes.
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UZILSD
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Post by UZILSD »

RnFR wrote:im not sure i understand what you are saying the reverse breakdown has to do with this whole equation. ii know that most of the characteristics on a diode datasheet are the max rating for voltage, current, temp, etc. but i'm not aware how those will fit into something like how they clip or rectify. is there some connection here? also i was aware of the different conduction knee points of diodes, but i don't think there is any proper way to measure this feature, is there? it doesn't seem to be something that is listed on a datasheet either. i'll try and do the experiment when i get a chance.

i could definitely hear the different capacitance of the diodes i tested as well. some of them seemed to cut a ton of low end, while others(Ge) seemed to pass plenty- especially when parallel.
Well, I laid down the crackpipe and I elaborated a theory for this.
I don't have an electronics degree so it's only guessins, but it may help.

Reading Ge diodes datasheets, it comes that Ge diodes have leaking currents in the uA range (microAmperes), while in silicons it's expected to be measured in picoAmperes, thousand time less circa.
In the circuit the current flowing in the final stage, after the diode, is also in the range of microamperes... you can tell doing some calculation: expecting a signal of some V peak to peak (say 4V for example), and having in this section an overall Z of 100K (assuming the Zin of the device connected after quite high), 4/100000=0.00004 A or 40 uA. So we are in the same current range and the currents interacts wtih each other.
Regarding the "noise pumping" effect, I would explain this way: between the capacitor and the diode after few cycles of signal a negative charge forms, until the voltage in that point reaches a negative value of the magnitude of V drop of the diode. When the guitar signal stops, since there is a negative voltage a DC current (the leaking current of the diode) begins to flow from the ground to the region, bringing with all AC signal with it (noise in this case). So the output is muted 'cause all AC noise, or part of it, flows to the more negative region. Once the negative charge is filled and the voltage returns to ground, no more current flow thru the diode and the noise reappears.

That's my 2 cents, don't take it like a scientific paper.

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Post by gus »

What you should try next is to double the output cap value and 1/2 the output cap value etc. keeping the same diode and input and output volume pot settings level. How does the sound change?

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Post by RnFR »

gus wrote:What you should try next is to double the output cap value and 1/2 the output cap value etc. keeping the same diode and input and output volume pot settings level. How does the sound change?

thanks gus. i'll give that a shot when i get a chance and report back.
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Post by SpencerPedals »

What are you using for cap values there RnFR? By that I mean which ones are pF vs. nF? They differ a bit from the Bosstone schematics I've seen. I just finished breadboarding a Companion and a Fuzzrite and a normal Bosstone was next on my list...but if you turned a semi-overdrive fuzz to a wall of fuzz sound, I'd like to hear it.

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Post by RnFR »

yeah, i've tweaked the cap values. that's pretty much the main thing i did besides the rectification at the end. all values are in uF except for the 470pF cap. it's obviously not finished yet, but it does sound pretty cool the way it is. try it with 3 Ge diodes in parallel where there is now one on the schem.

btw- the original bosstone isn't exactly tame to begin with, it's a pretty fierce little fuzz!
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Post by RnFR »

gus wrote:What you should try next is to double the output cap value and 1/2 the output cap value etc. keeping the same diode and input and output volume pot settings level. How does the sound change?
well i tried it. when the cap value is lowered, the compression artifacts are lessened, and when increased, the compression is likewise increased. this makes total sense, as less signal would result in less compression. it's kind of like a primitive leveling amplifier. i ended up putting a pot before the diodes and y-ing it out to two drastically different cap values. before the larger cap i added some resistance to make up for the smaller amount of signal on the other side of the pot, and to lessen the compression effect. i also tried a diode to ground along with the half wave rectifier, and that ended up sounding really nice and lessened any harshness that the rectifier might've brought on. i put a Ge diode to gr on the large cap side, and a silicon on the small side. now you can crossfade between a thick saturated tone and a thinner sharp silicon tone. the diode to ground ended up cutting a bit of volume, so i put a Ge make up stage after the diode configuration. it ended up sounding very thick, and ultra high gain- very cool. i'll post the schemo when i'm satisfied.

thanks for the tip gus! it ended up sparking a nice little tweaking session. :thumbsup
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Post by soulsonic »

I finally had time to try this today. I wasn't able to get a big thick sound, no matter what I tried. I think I didn't have a good choice of transistors - I wasn't able to find any PNP with a gain of more than 180 (and I tried several 2N2907...), so I think I'll try to scare up some higher gain transistors and give it another go tomorrow.
But, I was able to get the same results as RnFR with the different diodes on the output, ie; good, bad, high compression, low compression, etc... a green band 1N34A with a voltage drop of .67v definitely sounded better than a black band 1N34A with a voltage drop of .35v. In comparison, a 1N4001 with a voltage drop of .67V (same as the green 1N34...) didn't sound good at all. A BAT41 didn't barely make any sound, but a 1N5819 did (but didn't sound very good).
Putting several of the green band 1N34A in parallel did have an interesting thickening effect. One thing different I tried was to do two in parallel with the cathodes facing different directions (like you'd see in a normal clipping sort of setup) and this sounded decent to my ears.

But, overall, the fuzz I was getting in any case was fairly weak with a bad breakup on the top end. I think the transistors must have had something to do with that... I tried a stock Bosstone setup with same transistor pair and it sounded like useless junk, so I think the transistors I was using aren't good for this circuit. Need to go through my collection of unlabeled randoms and pick out some high gain PNP...
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Post by RnFR »

martin-

try putting a diode with the anode to ground at the anode of the rectifying diode "array". this really smoothed things out, but definitely has a different sound than typical clamps. try a silicon to ground or whatever to taste. also try some resistance before the rectifiers as well.
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Post by SpencerPedals »

I breadboarded it and it's a pretty large sounding fuzz. I used a 2907A with a hfe of 160ish and it worked alright but it does leave me wondering what it would sound like with a higher gain PNP, which I don't think I have, not being much for PNP stuff usually. It seemed a bit muddy to me, as it's currently breadboarded. I've only used diodes as called for in circuits; how do you measure voltage drop?

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Post by RnFR »

SpencerPedals wrote:I breadboarded it and it's a pretty large sounding fuzz. I used a 2907A with a hfe of 160ish and it worked alright but it does leave me wondering what it would sound like with a higher gain PNP, which I don't think I have, not being much for PNP stuff usually. It seemed a bit muddy to me, as it's currently breadboarded. I've only used diodes as called for in circuits; how do you measure voltage drop?
yes it is kinda muddy- actually i prefer sludgy :mrgreen: . that's pretty much what i was going for here- a giant wall of doom kind of sound. your meter should have a diode setting. that should get your voltage drop.
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Post by RnFR »

RnFR wrote:
SpencerPedals wrote:I breadboarded it and it's a pretty large sounding fuzz. I used a 2907A with a hfe of 160ish and it worked alright but it does leave me wondering what it would sound like with a higher gain PNP, which I don't think I have, not being much for PNP stuff usually. .
i've been using an unmarked pnp transistor in the same package as the 2n3565 and 2n5133- the name escapes me. it has an Hfe of around 350. i also tried a 2n5135 which was nice sounding as well.
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